Never mind the fact that any nukes used against North Korea would kill a whole lot of South Koreans as well. Not to mention a bunch of Chinese. Not to mention Japanese. Not to mention some Russians in the Vladivostok region.
And I'm betting that the Chinese, in particular, would not be very happy about this. They have way more nukes than North Korea.
And also, too, guess which way the wind blows and the ocean currents go? Well, that would be eastward. Which would mean that any radiation would wind up on our shores.